The invention relates to a method for cooling an electrolytic capacitor comprising a casing that surrounds a capacitive element.
The invention also relates to an electrolytic capacitor comprising a casing that surrounds a capacitive element.
With the increasing power density of electronic components, cooling has become more demanding. Increased power dissipation and high standards set by the environment of use have forced manufacturers in some cases to start using arrangements based on liquid cooling. One of the components creating problems in liquid cooling is the electrolytic capacitor.
The prior art teaches several alternative arrangements for cooling electrolytic capacitors via the outer casing. One arrangement relates to cooling the capacitor via the bottom plate (ACW6XX Drives Hardware Manual, 3AFY64314149). In such a case the bottom plate must be somehow insulated from the cooling circulation since the casing is in electrical contact with the cathode of the capacitor. This insulation creates problems in both assembly and heat transmission. Such an insulating layer should be easy to amount and in addition to the electrical insulating properties, it should have rather high thermal conductivity, which is very difficult to obtain with the aforementioned prior art arrangement.
Cooling can also be arranged from the inside of the electrolytic capacitors, since there is a pin in the middle of the capacitor, and the anode and cathode foils and the insulating layer are wound around the pin. Furthermore, the central pin also functions as a part of external fastening. This pin can be replaced with a cooling arrangement. Publication JP 1.1329899 discloses a capacitive element, where a metal foil and an insulating material have been coiled around a thermal pipe that also acts as the core or central pin. The end of the thermal pipe situated outside the capacitive element is provided with cooling fins, so that a cooling medium can be made to circulate in the thermal pipe. Thus in this case, the cooling medium is made to vaporize at the end situated inside the capacitive element and the medium is condensed back into a liquid at the other end located outside the capacitive element. Since the thermal pipe is known per se, the operation thereof will not be described in more detail herein. A problem with the arrangement according to the aforementioned JP publication is that it only transfers thermal energy away from the core of the capacitive element, whereas the thermal energy emitted by the thermal pipe should be further conducted out of the system so as not to heat the other components therein.
EP 0,780,855 discloses an electrolytic capacitor comprising a pipe or a cooling duct that travels through the capacitor in the middle of a capacitive element. A cooling medium, such as air, can be made to flow through the pipe. Since the pipe is not insulated at all from the electrolytic capacitor, the cooling medium in this known arrangement cannot be liquid, such as water. Another problem with the arrangement according to the EP publication is that since the pipe travels through the entire electrolytic capacitor, the structure of the capacitor cover must be changed due to the opening to be provided for the pipe in the cover. The pipe and the cover will be joined by a sealing joint, which makes it more difficult to provide electric insulation and causes one more joint in the structure, through which electrolytic liquid can flow out. A conventional electrolytic capacitor comprises a joint between the outer casing and the cover. It is difficult to close tightly two joints on the same plane, and sealing of the two joints requires that the joints must be exactly on the same level after the closing.